1. Field of the Invention
The invention relates to the field of compacting a granular, loosely coherent mass, such as soil-moist concrete cement. By compacting the mass, the air content thereof is reduced, and a strong coherent product is obtained. The products in question may be tiles, kerbstones and various other products made of concrete and the like. In particular, the invention relates to a compacting device with a hydraulic exciter device, and a hydraulic exciter device suitable for such a compacting device.
2. Description of the Related Art
NL-1005862 and EP 0 870 585 B1 disclose a device for compacting a granular, loosely coherent mass, such as soil-moist cement, for producing end products, such as tiles, kerbstones and the like. The known device comprises a vibrating table and a mould for the mass to be compacted, a stamp for pressing onto the mass in the mould, a hydraulic exciter and a hydraulic pressure element connected to the vibrating table or stamp, respectively, drive means and control means for controlling the exciter and the pressure element. The known device is configured so as to carry out a method which comprises the following steps:                selecting a frequency range with a lowest value and a highest value for the excitation frequency,        regulating the excitation frequency in such a manner that said frequency range is passed through at least partly and that the natural frequency of the hydraulic-mechanical mass spring system is reached, said hydraulic-mechanical mass spring system being formed by the movable part of the exciter, the vibrating table, the mould and the mass to be compacted, as well as the compressible hydraulic medium which is situated between the movable part of the exciter and the respective drive means (in particular comprising an electrohydraulic control element).        
The hydraulic exciter of the known device comprises a piston/cylinder device in which the movable part comprises a piston which is moved to and fro in an excitation cylinder by means of the respective drive means and control means. The piston is connected to a piston rod which is connected to the vibrating table at an excitation position on the vibrating table, which makes it possible to produce an excitation displacement at the excitation position. The excitation displacement is the sum of a working position and an excitation oscillation with the excitation frequency and an excitation amplitude around this working position.
It is a drawback of the known device that the vibrating table has to have a high degree of stiffness in order to achieve a substantially uniform vibration across the entire surface of the vibrating table when this is driven in the centre by the exciter. In order to produce larger products and/or more products per run, the dimensions of the vibrating table have to be increased. If the dimensions of the vibrating table are increased, the resulting table will be very heavy due to the stiffness requirements. This entire heavy table will have to be driven and pass through the frequency range which is associated with the great forces which are required therefor. It is therefore desired to provide a device which can be operated in a more efficient manner, for example due to the fact that it may comprise a relatively light table.
In a known device, the excitation cylinder is, for example, a piston/cylinder device fitted with hydrostatic bearings. Such a known piston/cylinder device is associated with, for example, an energy loss of approximately 20% due to leakage flow across the hydrostatic bearings. It is therefore desired to provide a more efficient excitation cylinder. Preferably, the more efficient excitation cylinder can in addition be used in the more efficient device, so that an optimum efficient device is obtained.
SU-A-856796 discloses a compacting device, comprising:
a vibrating table, a mould which is intended for the mass to be compacted and which is attached to the vibrating table, at least during use of the compacting device, a hydraulic exciter device connected to the vibrating table, wherein the hydraulic exciter device comprises a plurality of hydraulic exciters, each hydraulic exciter is connected to the vibrating table, the hydraulic exciters are mutually parallel, and the hydraulic exciter device is configured to drive the hydraulic exciters with excitation displacements of mutually substantially the same amplitude and the same frequency and in phase with one another.